The present invention relates to a method of preparing preforms for optical fibers. More particularly, the invention relates to a method to remove or significantly reduce undesirable refractive index variations in the central portion of the optical fibers.
One of the processes in manufacturing a glass optical fiber is to collapse a preform having a central duct into a solid glass rod. During the process of closing the central duct, volatile dopants, such as germanium, are desorbed or released from one location of the preform structure. These dopant molecules are then either redeposited at another location or transported out of the preform. Due to the redeposition and/or transportation of these dopants, an undesirable refractive index deviation at the center of the core is formed. FIG. 1 demonstrates a refractive index profile of a multimode glass optical fiber in the prior art. The refractive index deviation takes the form of spikes or dips. Such refractive index deviation is detrimental in high-speed systems with restricted launch conditions.
Hence, one has tried to solve this problem in the prior art. U.S. Pat. No. 4,793,843 discloses a method of manufacturing an optical fiber preform. In the process described therein, a gaseous etchant consisting of a combination of oxygen and a fluorocarbon compound C2F6 flows through a central duct of the preform when the preform is heated to collapse. That is, the etching occurs at the same time when the preform is collapsed. Hence, during the etching, the activated processes of fluorine diffusion into the glass structure and volatization of germanium occur because the etching is done at collapsing temperatures. U.S. Pat. No. 4,793,843 specifies that the central duct is approximately equal to one millimeter before the gaseous etchant is passed through the central duct. When the central duct size is small, the risk to cause the duct to close due to surface tension makes the process difficult to control. If the duct closes prematurely while forming the preform, it may cause the increase of the glass attenuation and undesirable airlines may be formed within the preform. Although the resulting solid preform and optical fibers drawn from the preform have an improved refractive index profile, U.S. Pat. No. 4,793,843 is limited to the etching with C2F6. Moreover, it is specifically disclosed that etching with SF6 at collapsing temperatures is not as effective as etching with C2F6 at the collapsing temperatures. In other prior art (e.g., U.S. Pat. Nos. 5,761,366; 4,557,561; and Scheneider et al., Proceedings of ECOC 1982) etching with SF6 during collapse is disclosed, but not as a separate step from collapsing.
The present invention provides a method to remove or significantly reduce undesirable refractive index variations in the central portion of the optical fibers. A method of preparing a preform for an optical fiber according to the present invention comprises the following steps. A preform having a central duct is heated at a first collapsing temperature to reduce the size of the central duct. The surface of the central duct is then etched at a temperature lower than the minimum collapsing temperature of the preform to remove a portion of deposited core materials. Finally, the preform is heated again at a second collapsing temperature to collapse completely the central duct of the preform. The etching step is performed by flowing an etchant gas through the central duct. The etchant gas comprises a mixture of oxygen and SF6. The temperature for etching is preferred to be about 200-400xc2x0 C. lower than the minimum collapsing temperature of the preform. The etching temperature is about 1600-2000xc2x0 C. in the invention.
In a first embodiment of the present invention, three collapsing passes are performed through a traversing torch in the direction from the preform intake to the preform exhaust at a temperature of 2150xc2x1100xc2x0 C. at decreasing torch traverse speeds in the first heating/partial collapsing step. The etching step is performed by flowing an etchant gas at about 1800xc2x0 C. through the central duct. The etchant gas comprises a mixture of 6 sccm of SF6 and 194 sccm of O2. Then, the preform is finally collapsed at a temperature of 2200xc2x1100xc2x0 C.
In a second embodiment of the present invention, three collapsing passes are performed through a traversing torch in the direction from the preform intake to the preform exhaust at a temperature of 2250xc2x1150xc2x0 C. at decreasing torch traverse speeds. The etching step is performed by flowing an etchant gas at about 1800xc2x0 C. through the central duct. The etchant gas comprises a mixture of 6 sccm of SF6 and 194 sccm of O2. In the final collapsing step, the preform is collapsed at a temperature of 2220xc2x1100xc2x0 C.
In a third embodiment of the present invention, three collapse passes are performed through a traversing torch in the direction from the preform exhaust end to the intake end while the exhaust end is plugged with a device to prevent gases from exiting the preform. Then, the device is removed from the exhaust end, and the etching step is performed by flowing an etchant gas of 6 sccm of SF6 and 194 sccm of O2 while the preform is heated at a temperature of 1800xc2x1150xc2x0 C. Finally, the preform is collapsed at 2200xc2x1100xc2x0 C. to form a solid rod.